“Have you been to see your doctor?” the woman asked her 72-year-old mother anxiously. Her mother had come from Miami to visit her in New York. They hadn’t seen each other for a couple of months, and her daughter hardly recognized her. Her mother had been slender; now, she looked emaciated. Her usually bright eyes peered dully above her newly prominent cheekbones.

But it was more than that — the unrelentingly cheerful, energetic, outgoing woman she had known her entire life had disappeared. Now her mother spoke of nothing but how awful she felt and spent most of her day in bed.

A Vacation Gone Wrong
It started a couple of months earlier when the mother and her partner were traveling in Italy for a month. While there, she began to feel irritable. She had fallen in love with this man eight years earlier — two years after the sudden death of her husband. And their years together had been happy ones. But on this trip, everything about him, about their relationship, began to grate on her. Suddenly she didn’t want to travel with him; she didn’t even want to see him. Indeed, she didn’t want to see anyone.

When she got home, she felt no better. She was a psychologist and recognized the symptoms of anxiety. She had never felt this before, but she had seen it in her patients. She went back to the psychiatrist she saw a few times after her husband died. Yes, the therapist agreed, she did seem anxious. She was also depressed. The woman accepted the psychiatric diagnoses, but told her therapist that it wasn’t just her mind; her body felt like it was too fatigued to do the work of living. But of course, the therapist told her, your mind is part of your body. People, especially older people, often feel symptoms of depression in their bodies — feeling sick and tired rather than sad.

The Medications Begin
The woman started taking an antidepressant and saw the psychiatrist once a week. When that didn’t help, the psychiatrist tried another drug. When she was still no better, she saw another psychiatrist, who added an antipsychotic. By the time the mother went to visit her daughter, she was on four medications: one for anxiety, two for depression and one for her insomnia. Yet she remained anxious, depressed and unable to sleep.

Upon returning home to Miami, she went to her doctor. When Dr. Mitch-Gomez saw her patient, she, too, was alarmed. She had lost weight. And she seemed to have lost so much more; she slouched in her chair as if it were too much work to sit up straight. During her routine physical five months earlier, the woman was her normal, lively self. Now she was a skinny, sullen shadow.

A Battery of Tests
The patient explained to Dr. Mitch-Gomez that she had suddenly developed anxiety and depression. She was taking four medications and meeting with a therapist, and still she felt terrible. She gave up her daily morning exercise class because she didn’t want to see anyone. She wasn’t suicidal, but she couldn’t bear feeling like this for the rest of her life.

She was one of the last patients of the day, so Dr. Mitch-Gomez settled in for a full investigation. The patient had a few vague complaints — she felt nauseated sometimes and occasionally awoke covered in sweat, as if she were going through menopause again. To Dr. Mitch-Gomez, it seemed clear that something beyond depression had to be going on. Although the patient was focused on the psychiatric symptoms, her doctor was worried about the fatigue, nausea, weight loss and sweat. The patient was treated for breast cancer nearly 15 years earlier. Could it have come back and invaded her liver, lungs or brain? It would be unusual after so many years, but not impossible.

Finally, a Clue
If not cancer, what? Hypothyroidism is common in older adults. So is Vitamin B12 deficiency. Each can cause depression. She frequently visited the Northeast; could this be some form of advanced Lyme disease? She sent the patient to the lab for blood tests and ordered a chest X-ray, a scan of her brain and another of her abdomen and pelvis.

The blood tests came back quickly. Her thyroid was normal; so was her vitamin B12. It wasn’t Lyme. The chest X-ray showed nothing. The head CT was completely normal. But the scan of the pelvis was not. There was an abnormality around the left ovary and uterus. A transvaginal ultrasound revealed a small mass on the ovary. After some testing, a gynecologist recommended removing the ovary and uterus.

The patient’s partner heard the news before she was even out of the O.R.: It was ovarian cancer, which had spread to a fallopian tube. But the surgeon assured him that they had gotten it all.

The patient no longer had cancer, but, she was still depressed. Dr. Mitch-Gomez referred her to a psychiatrist who specialized in cancer and depression. She wasn’t sure how the two were related — but it would be an odd coincidence, she thought, if they weren’t.

The Depression-Cancer Link
The patient went to see Dr. M. Beatriz Currier, an expert in the biochemical connection between cancer and depression. Patients with cancer are up to three times as likely to have depression as those without cancer, Dr. Currier told her new patient. It’s not simply that having cancer is depressing. It is that some cancers — perhaps most of them — can trigger the body to release chemicals that signal the brain to develop depressive symptoms. It’s a phenomenon first reported in 1931, when Dr. Joseph Yaskin, a neurologist at the University of Pennsylvania, published a case series on four otherwise-healthy middle-aged patients who were initially thought to have a late-onset depression and anxiety and who within months were found to have pancreatic cancer. The depression, Yaskin hypothesized, was a reaction “on the part of the central nervous system to the toxic or metabolic changes produced by the progressive visceral condition” — the cancer.

More recent research suggests that the body’s response to an injury like that caused by a tumor or an infection is to release messenger chemicals known as cytokines. These messengers communicate with other parts of the body — the immune system, the brain, the gut — to trigger a response to that injury. Different messengers elicit different responses, and the cytokines triggered by certain cancers have been found to set off not only a robust immune inflammatory response but also the neurological changes that cause depression. Some researchers hypothesize that the behavioral changes caused by depression and anxiety — lethargy and avoiding contact with others — might have provided a survival benefit in the face of infection or injury.

Your body released a flood of cytokines in response to the invasion of abnormal cells, Dr. Currier told her, and these chemicals triggered your brain to become depressed. For the first time in months, the patient felt a flicker of hope.

Hearing that she was depressed for a reason somehow made the depression a little easier to bear. With the cancer gone, she asked Dr. Currier, would her depression subside? That’s the theory, Dr. Currier agreed.

And it did, slowly, over months. During the last year Dr. Currier has weaned the patient off the psychiatric medications she was taking. She gained back her lost weight. She started going to her exercise class again. “I’m back,” she told me triumphantly — and it’s all thanks to her doctor, who suspected that what looked like a familiar depression could be something more.

Lisa Sanders, M.D., is a contributing writer for The New York Times Magazine and the author of “Every Patient Tells a Story: Medical Mysteries and the Art of Diagnosis.” If you have a solved case to share with Dr. Sanders, write her at Lisa.Sandersmd@gmail.com.